Embodiments of the invention generally relate to power generation systems and methods. More particularly, embodiments of the invention relate to cascaded fuel cell systems-based power generation systems and methods that include recirculation cycles, which can improve the overall efficiency of power generation.
Fuel cells are electrochemical energy conversion devices that have demonstrated a potential for relatively high efficiency and low pollution in power generation. A fuel cell generally provides a direct current (dc) which may be converted to alternating current (ac) via, for example, an inverter. The dc or ac voltage can be used to power motors, lights, and any number of electrical devices and systems. Fuel cells may operate in stationary, semi-stationary, or portable applications.
Certain fuel cells, such as solid oxide fuel cells (SOFCs), may operate in large-scale power systems that provide electricity to satisfy industrial and municipal needs. Others may be useful for smaller portable applications such as for example, powering cars. Common types of fuel cells include phosphoric acid (PAFC), molten carbonate (MCFC), proton exchange membrane (PEMFC), and solid oxide (SOFC), all generally named after their electrolytes.
In practice, fuel cells are typically amassed in electrical series, in an assembly of fuel cells (fuel cell) to produce power at useful voltages or currents. Therefore, interconnect structures may be used to connect or couple adjacent fuel cells in series or parallel. In general, components of a fuel cell include the electrolyte and two electrodes. The reactions that produce electricity generally take place at the electrodes where a catalyst is typically disposed to speed the reactions. The electrodes may be constructed as channels, porous layers, and the like, to increase the surface area for the chemical reactions to occur. The electrolyte carries electrically charged particles from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant.
Typically, the fuel cell converts hydrogen (fuel) and oxygen (oxidant) into water (byproduct) to produce electricity. The byproduct water may exit the fuel cell as steam in high-temperature operations. Hydrogen fuel may be provided via local reforming (e.g., on-site steam reforming) of carbon-based feedstock material, such as reforming of the more readily available natural gas and other hydrocarbon fuels and feedstock material. Examples of hydrocarbon fuels include natural gas, methane, ethane, propane, methanol, syngas, and other hydrocarbons.
The reforming of hydrocarbon fuel to produce hydrogen to feed the electrochemical reaction may be incorporated into the operation of the fuel cell. Moreover, such reforming may occur internal and/or external to the fuel cell. For reforming of hydrocarbons performed external to the fuel cell, the associated external reformer may be positioned remote from or adjacent to the fuel cell.
Fuel cell systems that can reform hydrocarbon internal and/or adjacent to the fuel cell may offer advantages, as described below. For example, the steam reforming reaction of hydrocarbons is typically endothermic, and therefore, internal reforming within the fuel cell or external reforming in an adjacent reformer may utilize the heat generated by the typically exothermic electrochemical reactions of the fuel cell. Furthermore, catalysts active in the electrochemical reaction of hydrogen and oxygen within the fuel cell to produce electricity may also facilitate internal reforming of hydrocarbon fuels. In SOFCs, for example, if a nickel catalyst is disposed at an electrode (e.g., an anode) to sustain the electrochemical reaction, the active nickel catalyst may also reform hydrocarbon fuel into hydrogen and carbon monoxide (CO). Moreover, both hydrogen and CO may be produced when reforming hydrocarbon feedstock. Thus, fuel cells, such as SOFCs that can utilize CO as fuel (in addition to hydrogen) are generally more attractive candidates for utilizing reformed hydrocarbon, and for internal and/or adjacent reforming of hydrocarbon fuel.
The exhaust components from fuel cells that operate at high temperatures can be directed to turbines and other types of engines, as part of a general combined cycle system. While such a system can be an attractive method for power generation, there are still some drawbacks present that can prevent wide-scale implementation. Some of the present day examples of fuel cells operating in simple-cycle mode, routinely achieve a conversion efficiency that is only about 50%. Thus there is a need for more efficient power generation systems that incorporate fuel cells.